1. Technical Field of the Invention
The present invention pertains in general to a method and apparatus for reducing code shift search time in a Global Positioning system receiver, and more particularly, to reducing code shift search time in a Global Positioning System receiver positioned within a cellular mobile station operating within a cellular telephone network.
2. Description of Related Art
It is desirable, and likely to be mandatory in the future, that cellular telephone networks be equipped to determine the geographical location of cellular telephones operating within the cellular telephone network. To meet this requirement it has been proposed that cellular telephones be equipped with Global Positioning System (GPS) receivers for determining the location of the cellular telephone. GPS receivers, however, are expensive, increase cellular telephone size, and consume the limited amounts of battery power available to the cellular telephone.
GPS receivers require auxiliary information also known as side information which refers to information such as the number of satellites in view, the Gold code number used by these satellites, the Doppler frequency for the signals transmitted by these satellites, the time delays from these satellites to users located at the center of the cell, and the size of the search window for the Gold code used by each satellite in view.
A typical GPS receiver includes at least a signal acquisition circuit and a demodulation and decoding circuit. Most of the complexity resides in the acquisition circuit. The acquisition circuit tries to search through all Gold codes used by the GPS system with frequency and timing uncertainty. The goal is to identify the code, symbol boundary and Doppler frequency used by the GPS satellites in view. This is a very complicated and power consuming process because it involves three-dimensional search (code-time-frequency). Once the receiver acquires the GPS signals, reading ephemeris data is very straightforward. When auxiliary information is provided to a GPS receiver, the complexity of such a three-dimensional searching process can be significantly reduced.
To calculate the auxiliary information for the GPS receiver, however, the approximate location of the GPS receiver must be known. Moreover, the closer the actual location of the GPS receiver to the location used in calculating the auxiliary information, the smaller the resulting location search to be performed by the GPS receiver. A smaller search greatly simplifies the time measurement process. The search can be reduced to finding the relative code shift position locations to much less than a one millisecond code cycle. Furthermore, once a code shift position is located for a first GPS satellite, the GPS receiver recalibrates its timing and corrects errors in the predicted code shift positions for the remaining GPS satellites. For a more detailed explanation regarding the use of auxiliary information by GPS receivers, reference is made to the previously identified cross referenced application.
Code shift search time also varies with the speed at which the GPS satellite is moving. As the GPS satellite moves, a Doppler frequency shift occurs whose magnitude is dependent on the speed at which the GPS satellite is moving.
It would be advantageous, therefore, to devise a method and apparatus to communicate the auxiliary information from a base station of a cellular telephone network to a GPS receiver located within a mobile station and further to provide the GPS receiver with satellite elevation information such that the GPS receiver can search the code shift position for a satellite having the highest elevation angle thereby reducing the time required for a GPS receiver to determine its location. It would still further be advantageous if the code shift search accommodated for a moving GPS receiver.
The present invention comprises a method and apparatus for communicating auxiliary information between a cellular telephone network and a GPS receiver positioned within a mobile station. A dedicated channel between the mobile station and the network is assigned by the network and uplink timing between the mobile station and the network is adjusted. The network requests auxiliary information, which among other information, includes satellite elevation angle information and upon receiving the auxiliary information, the network transmits the auxiliary information to the GPS receiver to perform a GPS code shift search at a common reference time and the GPS receiver then performs the GPS code shift search using the auxiliary information. The GPS receiver incorporates a bank of multiple correlators and accumulators which perform two dimensional searches of different time shifts and accumulators. The mobile station subsequently transmits time of arrival information to the cellular telephone network.